Conventional bicycle frames having a front triangle and a rear triangle require at least eight individual tubes which are secured together by welding, brazing or other fastening means to make up a usable frame assembly. The eight tubes are designated by their location relative to each other and to the other major components of a functioning bicycle. Thus, a short "head" tube supports the "head" axle joining the handlebar assembly and the front wheel fork, a "seat" tube supports the seat post and extends vertically as a column from the pedal axle support (the bottom bracket), the "top" tube connects the head tube and the top of the seat tube, the "down" tube connects the head tube and the bottom of the seat tube at the bottom bracket, a pair of "seat stays", one on either side of the rear wheel assembly, connects the top of the seat tube to the rear wheel supports, and pair of "chain stays", one on either side of the rear wheel assembly, connects the bottom of the seat tube to the rear wheel supports.
The multiplicity of connections required to secure these individual tubes and stays together, combined with the different forces encountered when the bicycle is ridden, may cause a variety of problems. For example, due to the many junctions between the tubes and stays and the nature of the fastening means required, it is not unusual for one or more of the joints or connections to fail in a manner that makes the entire frame unsuitable for use and incapable of economical repair. In addition, such frames may require many different tube shapes, further increasing the complexity of joining the shapes together, and may be relatively heavy due to the presence of the large number of structural elements and associated connections.
Conventional bicycle frames also may be of the monocoque type having a stressed, unitary outer skin assembled over stiffening ribs and other internal structural elements as illustrated by U.S. Pat. No. 4,513,986 issued Apr. 30, 1985. Instead of a unitary skin, monocoque frames may utilize two separate structural skin panels which extend parallel in spaced apart relation and are bonded to an internal core to form a three ply laminate such as illustrated in U.S. Pat. No. 4,230,332 issued Oct. 28, 1980. Although such monocoque frames may have some aerodynamic advantages over conventional triangular frames due to the aerodynamic shape of the outer skin, the unbroken plane of the outer skin may cause undesirable side wind sensitivity.
Both convention triangular frames and conventional monocoque frames have the disadvantage of requiring the manufacturing and fastening together of numerous structural pieces which is relatively time consuming and costly. The manufacture of monocoque frames may be particularly difficult and costly because of the large number of relatively small structural elements involved, including shear webs, reinforcing angles, reinforcing blocks, reinforcing strips and joining blocks, and these numerous elements after assembly must then be covered by a stress carrying structural outer skin.
Conventional triangular frames and conventional monocoque frames also may have too much compliance in torsion and lateral bending and too little vertical compliance. Increasing the torsional and lateral bending rigidity of conventional frames further reduces vertical compliance. This may result in relatively poor shock absorption and the excessive impact stresses. Unless these factors are properly balanced, bicycle handling and rider comfort are adversely affected and joints of the frame may be subjected during use to excessive stresses. The balancing of these factors is particularly critical for the frames of mountain bicycles of the type used for off road racing and touring over mountainous or other rugged terrain.
Another critical factor for mountain bicycles is the length of the load path between the rear wheel support means and the front wheel support means at the lower end of the front fork. Load paths of the lengths found in conventional bicycles are sufficiently long to significantly aggravate the stress loadings experienced by bicycles of the mountain type. Such aggravated stress loadings can cause early frame failure, particularly during off road racing. Furthermore, the complexity of conventional bicycle structures, either of the triangular or monocoque type, make a proper balancing of the foregoing factors extremely difficult to achieve as a practical matter.